This invention relates to the addressing of matrix array type ferroelectric liquid crystal cells. A feature of ferroelectric liquid crystal cells is that the permanent dipole moment of the liquid crystal interacts with an applied electric field and hence the response is different according to whether the applied field points in one direction, or points in the opposite direction.
A number of prior art matrix addressing schemes for such cells, including for instance that described in GB 2 173 629 A, have involved addressing on a line-by-line basis using unipolar strobe pulses applied sequentially to the members of a first set of electrodes to co-operate with bipolar data pulses applied in parallel to the members of a second set of electrodes. In the first scheme described in that specification the strobe pulses are of amplitude .vertline.V.sub.S .vertline. and duration t.sub.s, while the data pulses are charge balanced pulses, which make a first voltage excursion of amplitude .vertline.V.sub.D .vertline. and duration t.sub.s in one direction which is immediately followed by an oppositely directed voltage excursion also of amplitude .vertline.V.sub.D .vertline. and duration t.sub.s. The strobe pulses are synchronised with the first voltage excursions of the data pulses, and the operation of the addressing scheme is described on the basis that the magnitudes of V.sub.S and V.sub.D are chosen so that exposure of a pixel to a pulse of magnitude .vertline.V.sub.S +V.sub.D .vertline. for a duration t.sub.s is sufficient to effect switching of the pixel, whereas pulses of similar duration, but of amplitudes .vertline.V.sub.S -V.sub.D .vertline. or .vertline.V.sub.D .vertline., are insufficient to effect switching.
Although the switching stimulus is applied only for a duration t.sub.s, the data pulses are twice as long as this, and hence the line address time is 2 t.sub.s. Moreover, since the strobe pulse is unipolar, a single strobe pulse can switch pixels in one direction only.
The constraint of being able to switch pixels in only one direction in any one line address time means that complete freedom to change all the pixels of a display involves either erasure prior to writing, for instance a page erase before line-by-line writing, or writing a first field using strobe pulses of one polarity followed by the writing of a second field using strobe pulses of the opposite polarity.
Some examples of addressing schemes which use bipolar strobe pulses, and which in consequence are capable of switching pixels in either direction, are described in GB 2 146 473A. Thus the addressing scheme of that specification described with particular reference to its FIG. 3 uses a charge balanced bipolar strobe pulse which makes a first excursion of amplitude .vertline.V.sub.S .vertline. and duration t.sub.s in one direction, which is immediately followed by a oppositely directed voltage excursion also of amplitude .vertline.V.sub.S .vertline. and duration t.sub.s. This strobe pulse co-operates with charge balanced bipolar data pulses of similar format, but in which the excursions are of amplitude .vertline.V.sub.D .vertline. and duration 2 t.sub.s. The two voltage excursions of the strobe pulse are in this instance synchronised with the second voltage excursions of the data pulses. In this scheme the switching stimulus of magnitude .vertline.V.sub.S +V.sub.D .vertline. is applied only for a duration t.sub.s, but the data pulses are four times as long as this, and hence the line address time is 4 t.sub. s.